Most modern golf balls are based on peroxide cured polybutadiene. The co-agent of choice for this application is zinc diacrylate (ZDA).
Golf balls are available as solid and wound balls. Solid golf balls, which have all but replaced wound golf balls, provide maximum durability and distance. These balls have a core formed of a solid sphere of one or more layers. Typically, these balls have hard cores for high initial velocity and hard covers for low spin and durability.
A number of polymers, such as polybutadiene, natural rubber, styrene-butadiene, and isoprene, can be used in fabricating the solid cores. Today, solid core golf balls are predominantly made of high-cis polybutadiene. Moreover, in order to obtain the desired physical properties for golf balls, manufacturers have added cross-linking agents, such as metallic salts of an unsaturated carboxylic acid. The amount of cross-linking agent added is typically about 20 to 50 parts per hundred parts of polybutadiene. Most commonly, zinc diacrylate or zinc dimethacrylate are used for this purpose. Of these two cross-linkers, zinc diacrylate has been found to produce golf balls with greater initial velocity than zinc dimethacrylate.
For example, U.S. Pat. No. 5,998,506 to Nesbitt discloses a golf ball composition which includes cis 1,4-polybutadiene, an unsaturated polycarboxylic acid, a polymerization agent preferably consisting of dicumyl peroxide, and zinc diacrylate (ZDA) as the coagent.
Also, U.S. Pat. No. 6,218,453 to Boehm, et al. discloses a composition for the manufacture of a low spin golf ball comprising polybutadiene, a metal salt diacrylate, and a free radical initiator.
However, the zinc diacrylate traditionally used in golf ball compositions, displays many undesirable properties. It is in powder form and has a tendency to absorb moisture from the air. This causes it to compact, making dispersion of the ZDA powder into the polybutadiene difficult. The zinc diacrylate sticks to metal surfaces and causes mixing and cleaning problems. In addition, the ZDA powder is very fluffy, difficult to control and is a skin, eye and respiratory irritant. Manufacturers of zinc diacrylate have made the product in situ with zinc stearate in an attempt to reduce the dispersion problems, but these modifications have had little effect on the other physiological properties.
Predispersions of zinc diacrylate in EPDM and HNBR are already commercially available for use in other applications, but these polymers are not acceptable in golf balls.